The invention relates to a method for calculating an individual progressive lens.
Progressive lenses (also referred to as no-line bifocals, multifocal lenses, etc.) are usually understood to refer to lenses which have a different (lower) refractive power in the area through which the lens wearer observes an object at a greater distance-hereinafter referred to as the far part-than in the area (near part) through which the user observes a near object. The so-called progression zone is between the far part and the near part is where the effect of the lens increases continuously from that of the far part to that of the near part. The value of the increase in effect is also referred to as addition.
As a rule, the far part is located in the upper part of the lens and is designed for looking “into an infinite distance” while the near part is located in the lower area of the lens and is designed for reading in particular. For special applications, e.g., pilot's glasses or glasses for working at a computer monitor may be cited as examples here the far part and the near part may also be arranged differently and/or designed for different distances.
Furthermore, it is possible for a lens to have multiple near parts and/or multiple far parts and corresponding progression zones accordingly.
In progressive lenses having a constant refractive index an increase in refractive power between the far part and the near part requires that the curvature of one or both surfaces must change continuously from the far part to the near part.
The surfaces of lenses are usually characterized by the so-called principal radii of curvature R1, R2 at each point on the surface (sometimes instead of the principal radii of curvature, the so-called principal curvatures K1=1/R1 and K2=1/R2 are also given). The principal radii of curvature together with the refractive index n of the lens material determine the values often used to characterize a surface optometrically:Surface refractive value D=0.5*(n−1)*(1/R1+1/R2).Surface astigmatism A=(n−1)*(1/R1+1/R2).
The surface refractive value D is the value which achieves the increase in effect from the far part to the near part is achieved. The surface astigmatism A (the cylinder effect) is an “interfering property” because, unless the eye itself has an astigmatism to be corrected, an astigmatism exceeding a value of approx. 0.5 dpt leads to an image on the retina which is perceived as blurred.
WO 01/81979 describes a method for calculating a progressive lens by using the properties of refractive value and astigmatism which are determined along a line (principal line) which corresponds to the principal line of sight.
For the calculation of individual lenses, the optimization must be performed within a very short period of time because due to the great variety of possible combinations of effect, they can be calculated only on order.